Abstract: In this paper considerations are given to the following two situations which have not been taken into account by previous investigators: 1. There exists in general an outside unperforated zone, which, though narrow, has a noticeable effect on the stress distribution of tubesheet. 2. As the middle planes of the tubesheet and shell flange are not in the same plane, membrane forces, increasing largely the rotational rigidity of the shell flange and producing remarkable effect on stress distribution, will come into play. Numerical results by the proposed method of analysis check closely by experiments on three　tubesheets with triangular hole pattern, if the full tube thickness is considered in the ligament efficiency calculation.